MOORPARK, Calif. (AP) — More than two decades ago, two water distributors came up with a tantalizing idea to increase reserves in parched Southern California: Create an underground lake so vast it could hold enough to blanket Los Angeles — all 469 square miles — under a foot of water.

The reservoir deep within the earth would be injected with water imported from the snowy Sierra Mountains and other distant sources, which could be pumped back to the surface when needed to soak avocado and lemon groves and keep drinking fountains, espresso machines and toilets gurgling.

Officials boasted the subterranean vault would become a model for preserving scarce supplies and combatting droughts, not just in California but globally. Instead, clusters of wells and skeletal metal piping stand as a cautionary and costly reminder that the promise of water, the fuel of California’s economy and growth, can be as permanent as a mirage.

“They had a great vision,” said Reddy Pakala, who as Ventura County’s water director was expecting to benefit from the 18-mile-long trough running below orchards, tree farms and ranches. “It doesn’t work the way they told us it would work.”

The Las Posas Basin Aquifer Storage and Recovery Project, about 50 miles northwest of Los Angeles, illustrates the risks of what’s known as groundwater banking — warehousing water below ground in aquifers to pump out during dry spells or emergencies.

Imagine a bathtub beneath the earth. If geologic conditions are right, water can be added to such a basin, supplementing naturally occurring groundwater and creating a reserve for future use, much like a water tank.

But at Las Posas, water instead disappeared.

The project, now owned solely by wholesaler Calleguas Municipal Water District in Ventura County, promised to raise local groundwater levels up to 300 feet. Yet groundwater levels dropped steeply when the system went into operation, potentially threatening supplies for nearby residents, ranches and businesses that also draw water from the basin.

“The teacup appears to have a leak in it,” said Robert Eranio, general manager of the nearby Crestview Mutual Water Co., a Calleguas buyer that pipes water to homes.

An Associated Press review of government documents, along with dozens of interviews, found that the venture was marred by insufficient research, poor judgment and hollow assurances — all with a hefty price tag for ratepayers. The cost, estimated in 1995 at $47 million, has gradually tripled to about $150 million. More than $56 million in long-term debt remains.

Calleguas touted the project in the fall of 2010 as a “resounding success” that raised groundwater levels. And yet a year earlier, the company knew the aquifer wasn’t working properly. Not long after the declaration of “success,” the primary financial backer pulled out after Calleguas didn’t deliver promised water.

Where once officials projected the basin could hold nearly 100 billion gallons — ready for distribution with the flip of a pump switch — the reserve is now only about one-tenth of that, according to officials.

“What was known about the basin wasn’t the whole picture,” said Calleguas general manager Susan Mulligan. “It doesn’t work for a drought. The basin doesn’t store it.”

The project was announced formally in 1991 with the promise that the reservoir could hold enough for a roughly three-year supply for the more than 600,000 people who use Calleguas’ water, which is sold to cities, local agencies and companies that in turn provide water for residents, ranches and businesses.

According to company documents, Calleguas boasted that the project would “virtually drought-proof” its service area. The basin also was meant to resolve a vulnerability for the company: Almost all of Calleguas’ water is imported through a single pipeline, meaning customers could be left dry if an earthquake or other disaster caused serious damage.

Early signs were encouraging. Technical studies beginning in the late 1980s and a later pilot project concluded Las Posas could hold huge amounts of water. However, documents show, researchers also found unusual fluctuations in water levels. During one injection test, levels first increased — then dropped — in monitoring wells.

Researchers later discounted those findings. And it was only after most of the project had been built that serious problems began to appear.

The Metropolitan Water District of Southern California, a giant wholesaler that provides drinking water to nearly 19 million people, provided much of the project’s early funding. But the agency pulled out and was paid $54 million to cover its costs after Calleguas failed to deliver promised water from the aquifer — first in 2009 as drought gripped the state and then again in 2010, when Calleguas came up nearly 2 billion gallons short.

Calleguas officials say their battery of pumps can be used for short periods and longer in an emergency, but at a price. Some fluctuation in groundwater levels is expected. But internal documents state that if Calleguas pumps at a heavy clip for an extended period “water levels will drop by hundreds of feet.”

“Many of the geological assumptions and groundwater modeling criteria related to the facility’s operation were in error,” a draft report prepared by Calleguas officials concluded last year.

Some speculate Calleguas never injected enough water into the basin before making withdrawals, while others suspect ranchers and other local pumpers sucked off the surplus created by the company, knowingly or not.

The project is now relegated to limited use, including during brief, annual maintenance outages when Metropolitan briefly cuts its supply. However, Calleguas must pump water into the wells before any water is withdrawn, as a hedge against possible water drops.

Mulligan acknowledged that more testing was needed early on but said the project was still a worthwhile venture. The company continues to study the basin, and has hired a groundwater manager to assess the problems.

Home to 38 million people, California wouldn’t exist without a vast array of reservoirs, aqueducts, canals, pipelines and pumping plants that distribute water.

Much of Southern California is a desert-like climate. Snowfall can top 500 inches in the craggy Sierras on the state’s eastern edge, but only about 15 inches of rain fall in the Los Angeles area in an average season — and that can drop to single digits in dry ones.

This year, the state recorded its driest January through March since record-keeping began in the mid-1890s, portending another year of shortages.

Underground storage has worked before. As far back as the 1950s, water imported from the Colorado River was stashed in local groundwater basins. There are dozens of the hidden chambers in California. The Metropolitan district has withdrawn over 234 billion gallons from Kern County water banks since 2001, according to one estimate.

As for the Las Posas project: “It’s a shame … no question about it,” said Ellen Hanak, senior policy fellow at the Public Policy Institute of California, which has closely studied water banking. “It’s true in water that sometimes you try stuff, and it’s not going to work out.”

David Schwabauer, who tends avocados and lemons on a 750-acre ranch a short drive from the Las Posas pumps, was bullish on underground storage, seeing it as a way to guarantee water whenever supplies got tight.

For years he’s watched groundwater levels retreat with higher demand from encroaching development, forcing ranchers and farmers to sink piping deeper into the earth or drill expensive, new wells for irrigation. In the heat of summer, he pumps 3,000 gallons a minute for his thirsty trees.

Once hopeful for a safeguard against Mother Nature, Schwabauer is frustrated knowing so much was spent for so little.

“There’s not an hour that goes by that I’m not thinking about water,” said Schwabauer, who served on a local agency that supported Las Posas. But underground storage is “much more complex, we know today, than we did in the beginning.”

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